1. Field of the Invention
This invention relates to process for producing an extruded, polystyrene resin foam for use as a heat insulator for walls, floors, roofs and so on of buildings or as a tatami mat core. The present invention is also directed to an extruded, polystyrene resin foam.
2. Description of Prior Art
Because polystyrene resin foams have excellent heat insulating property and desirable mechanical strengths, plates thereof have been widely used as heat insulators.
One method of producing such a foam plate includes the steps of heating and kneading a polystyrene resin together with a nucleating agent, mixing the kneaded mixture with a physical blowing agent, extruding the resulting melted mixture (foamable composition), and optionally feeding the extrudate to a shaping device attached to the outlet of the die (see, for example, Japanese Unexamined Patent Publications No. 2003-292664, No. 2004-59595 and No. 2004-196907).
Japanese Patent No. 3,244,196 discloses an extruded polystyrene resin foam sheet obtained by extruding a polystyrene resin having a ratio (Mz/Mw) of the Z average molecular weight Mz to the weight average molecular weight Mw of 1.7 to 2.5, a ratio (Mw/Mn) of the weight average molecular weight Mw to the number average molecular weight Mn of 2.0 to 3.0 and a weight average molecular weight of 2.8×105 to 4.0×105, each measured by gel permeation chromatography. The foam sheet is used for the production of shaped articles by thermoforming. The above method, however, cannot produce a thick foam. The patent is silent with respect to a Mz/Mn ratio. The Mz/Mn ratios of the polystyrene resins specifically disclosed in the working examples of the patent are calculated to be 2.4 and 4.7.
In order to obtain a thick foam having low density (high expansion ratio), it is necessary to use a large amount of a blowing agent. The use of a large amount of the blowing agent, however, causes separation of the blowing agent from the foamable composition and premature foaming of the foamable composition within the die lip prior to the discharge thereof from the die lip. As a consequence, the resulting foam becomes poor in appearance and in uniformity of cell sizes.